Clamping device for lifting slab, panel or sheet material

ABSTRACT

Clamping device suitable for lifting and handling of sheet like-objects, having a rigid frame with a fixed jaw that provides a first clamping surface, a movable jaw that provides a second clamping surface that is substantially parallel to the first clamping surface, the movable jaw being supported at the frame for reciprocating, linear movement towards and away from the fixed jaw, and an operating mechanism arranged to bias the movable jaw towards the fixed jaw in response to an actuating force being exerted on a tensile force transmitting member of the operating mechanism, thereby to abut the respective clamping surfaces onto opposite faces of an object received between the jaws and frictionally clamping same against displacement, characterised in that the movable jaw comprises a clamping plate on which the second clamping surface is provided, the clamping plate being inclined at an angle of about 8 to 12 degrees with respect to the first and second clamping surfaces and being supported at the frame to maintain said angle during movement to and from the fixed jaw, and in that the tensile force transmitting member is guided and supported at an upper location of the clamping plate that substantially aligns with or overhangs the second clamping surface.

FIELD OF THE INVENTION

The present invention relates to clamping devices that may find use inhoisting and handling heavy slab, panel or sheet materials such asstone, masonry, concrete, marble, metal and the like materials.Specifically, the invention relates to an improvement for lifting clampsof the type having a rigid frame having a fixed jaw plate that providesa first clamping surface, a movable jaw plate that provides a secondclamping surface and which is supported at the frame for reciprocatingmovement towards and away from the fixed jaw plate, and an actuatingmechanism to bias the movable jaw plate towards the fixed jaw platewhereby the respective clamping surfaces abut onto opposite faces of thesheet material and frictionally clamp same against displacement.

BACKGROUND OF THE INVENTION

Heavy slabs and sheets of material are usually stored and stackedupright standing or in slightly inclined orientation. Handling of thistype of materials often entails use of lifting clamps that grip thesheet (hereinafter used generically to also encompass slabs and otherplanar objects) at its upper edge for hoisting. Consequently, it isconvenient in the following description of known devices and the liftingdevice in accordance with the present invention to use referenceterminology such as ‘vertical’, ‘horizontal’, ‘upper’, ‘lower’ andsimilar when describing operation and components of such clampingdevices, bearing in mind that these devices may also be used in a‘horizontal’ or other orientation, eg as a simple clamp or a haulageattachment. Thus, unless otherwise clear in the context, such referenceterms are not to be interpreted as a limitation.

A lifting clamp of the type with which the present invention isconcerned is known from U.S. Pat. No. 5,893,595 (Corbett). The liftingdevice includes a rigid frame comprising vertical, parallel spaced apartside plates, the upper ends of which are rigidly secured together byfour tubular cross-members. The lower portion of one of the side platesis angled away in downward orientation from the other side plate, thelatter providing a fixed clamping jaw of the device. A verticallyextending plate is mounted for horizontal sliding movement on thecross-members between the side plates and provides a movable jaw of thedevice. An actuator carriage which is disposed for guided vertical upand down movement is located between the movable jaw and the lower,angled portion of the frame side plate, whereby a set of rollers of thecarriage respectively engage the facing surfaces of the movable jawplate and the angled portion of the frame side plate. The carriage isconnected to a strip member that extends beyond the upper end of theframe and has a lifting lug to which is attachable a lifting cable orchain. In order to lift (or otherwise handle) sheet material, the deviceis placed over the upper edge of the sheet so that it is receivedbetween the fixed and movable jaw plates, the carriage is raised bylifting the strip member through pulling the lifting cable upwards,whereby the carriage travels on the angled frame side plate portion anddisplaces the movable jaw horizontally until it abuts on the facingsurface of the sheet material. Upon increasing the upward pulling force,the sheet material is frictionally clamped sufficiently tightly for itto be lifted with the device. In essence, clamping of the sheet materialbetween the plate jaws is achieved by wedging the carriage between theframe side plate and the movable jaw, and the clamping force ismaintained for as long as there is upward force being on the liftingcable.

A lifting clamp of similar design to that of Corbett is disclosed inGerman patent publication DE 199 23 788 A1 (Scheibenbogen GmbH & Co KG),where, however, the mechanism employed to move the movable jaw intogripping engagement with the sheet material comprises a cam pulleyinstead of a wedging carriage. The pulley is mounted on the movable jawplate with its axis of rotation perpendicular to the plane of themovable jaw plate, four rollers serving to support the pulley inparallel relationship at the facing jaw surface. Two cams are arrangedon the opposite face of the pulley, concentrically with and symmetricalabout the rotation axis. The cams extend equiradially over a sector ofthe of the pulley circumference. The hight of the cams increase fromnear the pulley face to a maximum hight that is dictated by the maximumspacing between the fixed and movable jaws of the clamp in their fullyspaced apart position. The cams thus provide sloping guide and bearingsurfaces for respective actuator rollers that are secured in fixedrelationship on the frame plate of the device that faces the pulley. Anactuator cable that is secured to the pulley perimeter is used to rotatethe pulley, the actuator cable advantageously terminating in a hoop onwhich a hoisting cable or chain may be attached. In operation of theclamp, when a pulling force is exerted on the actuator cable, rotationof the pulley causes the actuator rollers to travel along the inclined(or curved) bearing surfaces of the cams, thereby displacing the movablejaw away from the facing frame plate towards the fixed jaw and clampingsheet material received between the jaws.

One disadvantage that has been observed with lifting clamps made inaccordance with Corbett as well as Scheibenbogen is the tendency of theclamp frame, on lifting of in particular thin sheet material from theground, of rotating into a different spatial attitude from the initiallygiven “no load” attitude, the later being characterised by thesubstantially vertical orientation of the clamping surfaces of the jawswhen these are clamped onto a sheet of material resting upright on theground. This rotation induces swaying and swinging of the sheet materialat the lifting cable that makes precise hoisting difficult, andincreases breakage risk upon hitting against objects in the vicinity ofthe sheet. The moment that causes this rotation is induced by thepresence of an out-of-alignment force pair on initial lifting off of thesheet attached to the clamp frame (upward directed lifting force vsdownward directed weight of sheet material and clamp). This movementinducing moment decreases and ceases as the clamp frame and the sheetmaterial rotate into and eventually assume a final, slightly inclinedorientation with respect to the vertical, as the respective centres ofgravity of the clamp and the sheet material seek to and ultimately alignthemselves along the same (vertical) line along which the lifting forceis being exerted.

This problem stems from the lay-out of the clamp as such, ie due to thepresence of a fixed clamp against which a single movable clamping jawplate is forced. With such lay out, the line along which gravity forceacts on the upright sheet material locates within the clamp frame atdifferent traverse locations, depending on the thickness of the sheet,and does not align with the point at which the upward directed hoistingforce acts on the clamp frame, notwithstanding the lifting forceultimately acts on the carriage or cam pulley of the clamp actuatingmechanism in close vicinity of the secured sheet material.

Equally, whilst the cam pulley clamping mechanism of Scheibenbogen hasthe advantage over Corbett's of providing a more even distribution ofclamping pressure onto the sheet material, the Scheibenbogen liftingclamp has a more pronounced tendency of swaying under load, whichresults in difficulties in controlling movement of sheet material duringtransport.

One object of the present invention is to provide a lifting device ofthe aforementioned type in which the tendency of the clamp of swayinginto an inclined position during lifting of sheet material is minimised.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention there is provided aclamping device suitable for lifting and handling of sheet like-objects,having a rigid frame with a fixed jaw that provides a first clampingsurface, a movable jaw that provides a second clamping surface that issubstantially parallel to the first clamping surface, the movable jawbeing supported at the frame for reciprocating, linear movement towardsand away from the fixed jaw, and an operating mechanism arranged to biasthe movable jaw towards the fixed jaw in response to an actuating forcebeing exerted on a tensile force transmitting member of the operatingmechanism, thereby to abut the respective clamping surfaces ontoopposite faces of an object received between the jaws and frictionallyclamping same against displacement, characterised in that the movablejaw comprises a sliding plate on which the second clamping surface isprovided, the sliding plate being inclined at an angle of about 8 to 12degrees with respect to the first and second clamping surfaces and beingsupported at the frame to maintain said angle during movement to andfrom the fixed jaw, and in that the tensile force transmitting member isguided and supported at an upper location of the sliding plate thatsubstantially aligns with or overhangs the second clamping surface.

It has been found that the inclined arrangement of the sliding plate andlocation of the force transmitting member reaction point at the slidingplate have positive effects in minimising the presence of swing-inducingmoments when a slab or sheet material of relatively small thickness ishoisted with the clamp, as the reaction point of the lifting force atthe sliding plate is brought in closer alignment with the line alongwhich gravity exerts force on the sheet material. An inclination angleof the sliding plate of about 11 degrees renders particularly goodresults in minimising the gravity and lifting force induced moment atthe clamp frame, providing thus better lift/hoisting control.

Advantageously, the operating mechanism of the lifting clamp comprises acam pulley system whose operating principle (but not necessarily itsconstructional layout) is similar to the one described in abovementioned German document, the contents of which is incorporated hereinby way of cross-reference. Such actuating system provides for a moreuniform and even clamping load distribution as compared to the solutionof Corbett.

Accordingly, in accordance with a preferred implementation of theinvention, the operating mechanism includes a drive pulley that ismounted on the sliding plate of the movable jaw with its axis ofrotation perpendicular to the plate plane, a plurality (preferably fouror more) of bearing rollers arranged to support the pulley in parallelrelationship at the facing sliding plate surface, at least one,preferably two actuating cams protruding from the pulley surface thatfaces away from the sliding plate, the actuating cams beingsymmetrically disposed about the rotation axis of the pulley andextending along an arc sector radially inwards of the pulleycircumference, the cams having a hight that increases from near thepulley face to a maximum hight that is related to the maximum spacingbetween the fixed and movable jaws of the clamp in their fully spacedapart position, the cams each defining a sloped guide and bearingsurface for respective actuator rollers that are secured in fixedrelationship on a stationary part of the clamp frame that faces thepulley, the arrangement being such that upon rotation of the pulley, theactuator rollers travel along the guide and bearing surfaces of the camsthereby displacing the pulley and therewith associated sliding platealong the axis of rotation of the pulley relative to the stationaryclamp frame part.

The tensile force transmitting member used to rotate the pulley ispreferably an actuator cable that is suitably secured to the pulleyperimeter and partially wound on and running in a peripheral groove ofthe pulley. The actuator cable may advantageously terminate at itsother, free end in a hoop on which a hoisting cable or chain may beattached.

In operation of the clamp, when a pulling force is exerted on theactuator cable, eg when hoisting a slab, rotation of the pulley causesthe actuator rollers to travel along the bearing surfaces of the cams.The hight increase of the cams can be constant, progressive ordecreasing, ie the contour of the bearing surfaces can be rectilinear,curved or otherwise shaped, thereby to achieve a desired displacementmovement of the sliding plate away from the stationary frame part, eg aframe plate, towards the fixed jaw. The sheet material received betweenthe jaws is thus clamped with a force that is a function of the pulleydiameter, the distance of the arc-shaped cams from the axis of rotationof the pulley, and the weight of the material being hoisted with theclamp.

When employing such cam pulley operating mechanism, the rigid frame maypreferably be an assembly consisting of a side plate that provides thefixed jaw, a substantially rectangular parallelepiped shaped housingpart with an open side disposed to face the side plate, whereby a bottomwall or side walls of the housing may provide the stationary frame partthat supports and locates the actuating rollers that engage the campulley, and a plurality of (preferably four) cross struts rigidlyjoining the side plate and the housing part in spaced apart relationship at upper ends thereof, the housing part having a cavity in whichthe cam pulley of the operating mechanism locates when the clamp is in afully open state, eg non-clamping state, in which the movable slidingplate comes to rest adjacent the housing part and covers the open sideface. The partially encased operating mechanism is thus protected fromdamage to its moving parts.

In a further preferred embodiment, the clamping device may incorporate alocking mechanism that is arranged to lock the cam pulley againstrotation. The locking mechanism may be designed to be engageable onlywhen the movable jaw is in its non-clamping position, or in anyposition.

One embodiment of the locking mechanism includes a rotatable lockingplate attached to or otherwise operably connected to a handle, knob orother manual actuating member, the locking plate being supported at thehousing part in a location where it can be rotated or displaced in andout of engagement with a locking receptacle provided either on the campulley or a support shaft thereof. A spring-loaded actuator rod thatacts on the latch mechanism, located at the housing part front or sidewall, is preferred.

A preferred embodiment of a clamping device as used for lifting slabmaterials in accordance with the present invention, and further featuresand advantages of the invention, will be described with the accompanyingdrawing. dr

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a side view of a lifting clamp in accordance with a firstembodiment of the invention, its gripping position with a sheet materialsuspended there from;

FIG. 1 b is a front elevation of the clamp of FIG. 1 a;

FIG. 2 is a longitudinal section of the clamp along line A-A of FIG. 1b;

FIG. 3 is a plan section of the clamp taken along line B-B of FIG. 2;

FIGS. 4 a to 4 c are, respectively, a front elevation, a side view and asection along line D-D of a housing part of the clamp that provides astationary clamp part and protective cover for the operating andclamping mechanism of the clamp device of FIG. 1 a;

FIGS. 5 a to 5 c are, respectively, a front elevation, a side sectionalong line E-E and a side view of a sliding plate that forms part of amovable jaw member of the clamp device of FIG. 1 a;

FIG. 6 shows in cross-section a partial view of the cam pulley of theoperating and clamping mechanism for actuating the movable jaw of theclamp device;

FIGS. 7 a and 7 b are an isometric top and bottom view of the cam pulleyof the operating and clamping mechanism of the clamp device; and

FIG. 8 is a plan section similar to FIG. 3 of the clamp deviceincorporating a second embodiment of a cam pulley locking device inaccordance with an aspect of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT

In the following description, reference terms such as ‘upper’, ‘lower’,‘vertical’, ‘horizontal’, ‘left’ and ‘right’ are chosen with regard tothe drawing plane of the relevant figures and to aid in referencingclamp components with respect to one another.

FIG. 1 a illustrates a clamping device 10 in accordance with the presentinvention as used for lifting and hoisting sheet-like materials 15 ofsubstantial dimensions and weight. The expression “sheet materials” ishere generically also used herein to describe slabs of stone, marble,sheet or sheets of metal tightly bundled into a stack, or the like.

The lifting clamp 10 illustrated in greater detail in FIGS. 1 b to 3comprises substantially three subassemblies or groups, a fixed,stationary jaw section 20, a movable jaw section 22 and a stationaryhousing section 24, wherein an actuating mechanism 90 employed to movethe movable jaw section 22 with respect to the fixed jaw section andhousing section 24 is located between the housing and movable jawsections.

Jaw section 20 and housing section 24 define a rigid clamp frame and arejoined together at an upper end in spaced apart relationship by means of4 cross bars 26 and 28, the terminal ends of which are fixedly securedto the respective sections 20 and 24 by appropriate screws 30, 32, seebelow. The upper pair of cross bars 28 also serve to suspend in guidedmanner the movable jaw section 22 so that it may be caused to slidethere along upon actuation of the operating and clamping mechanismdescribed below toward and away from the stationary jaw section 20 ofthe device.

Stationary jaw section 20 is comprised of a substantially rectangularsteel plate member 34, having four bores disposed on an upper section toreceive and secure the screws for fixing the cross bars 26, 28 to it. Arubber glove 36 is fitted to the lower portion of steel plate member 34.As shown in FIG. 2, the rubber glove 36 has a trapezoidal cross-sectionwith a lower portion that is angled slightly inwardly to allow for easymanoeuvring of the jaw 20 onto the work piece or slab and into grippingposition for lifting. Glove 36 provides a high friction material betweenthe stationary jaw and the slab being lifted to prevent the slab fromslipping or being damaged during lifting operation.

As best seen in FIGS. 4 a to 4 c, housing section 24 includes asubstantially rectangular parallelepiped shaped housing box 38, that hasa solid material upper portion 40 that is angled in side view about 9 to12, preferably 11 degrees with respect to a lower housing portion 42.Upper portion has formed therein four bores of stepped diameter thatserve to form-fittingly receive the terminal ends of cross rods 26 and28 and secure same against displacement therein using screws 30, 32(FIG. 2). Lower portion 42 includes an open cavity 44 bordered byopposing side walls 50, a bottom wall 51, a base wall 48 and said uppersolid material portion 40. Within cavity 40 are arranged twodiametrically opposing bearing pedestals 52 a and 52 b that are madeintegral with the respective side walls 50 and the base wall 48 andprotrude therefrom into the cavity 40. Pedestals 52 a and 52 b serve torespectively support an actuator roller 86 (see FIGS. 2 and 3) of theoperating mechanism for the movable jaw 22 (described below). Notillustrated in greater detail, actuating rollers 86 are supported onrespective shafts that extend substantially parallel to base wall 42,the shafts being secured in known manner to the pedestals. Cavity 44 inlower housing portion 42 is dimensioned such as to cover and receivetherein a cam pulley 94 of the operating mechanism 90 described below. Aprotective rubber shoe 54 covers the lower terminal end of housing box38.

As can be best gleaned from FIGS. 1 a and 2, housing section 24 alsocarries the components of a locking or arresting mechanism 65 employedto lock the movable jaw section 22 of the clamp against axialdisplacement along cross rods 28, whilst an additional arrestingmechanism 60, described in greater detail below, serves to lock theactuating mechanism 90.

Turning next to FIG. 2 and 5a to 5c, the movable jaw section 22comprises a sliding plate (assembly) 70 that includes a generallyrectangular base plate 72 to which are welded at the upper width-wiseedges thereof two side plates 73, 74 that extend perpendicular to baseplate 72. Parallel to the latter and received between side plates 73, 74is welded a smaller face plate 75. Guide tubes 76 that extend parallelto one another are received in openings in the face and base plates 72,75 and welded to the plates. Ring seals 78 and a guide bush 79 aresecured within the guide tubes as illustrated in FIG. 5 b.

Guide tubes 76 serve the purpose of guiding and suspending the slidingplate assembly 70 in a defined orientation at the upper pair of crossrods 28 that extend between fixed jaw section 20 and housing section 24,that is with the plate assembly 70 maintaining an inclined attitude withrespect to a vertical line as shown in FIGS. 1 a and 2 whilst itdisplaces along the horizontally orientated cross rods (bars) 28. Thisangle is preferably set to about 11 degrees. Heretofore, the tubes 78are welded to the base plate 72 with their longitudinal axis inclinedabout 11 degrees with respect to a line that extends orthogonally withrespect to the plate's plane. As can be further seen from FIGS. 5 a and5 b, an additional pair of openings 76 a and 76 b is respectivelyprovided in face and base plates 72 and 75 below the guide tubes 76thereby to allow the lower pair of cross rods 26 to pass extend throughthe sliding plate assembly 70 with play, as seen in FIG. 2.

As best is also seen in FIG. 2, a clamping plate 77, which inlongitudinal section is about triangular, is mounted in fixed manner tothe lower portion of base plate 72 of sliding plate assembly 70. Theclamp plate 77 is rubber coated and defines the clamping surface of themovable jaw section 22, wherein the inclination angle at the pointedupper end of clamping plate is about 11 degrees, such that theinclination angle maintained between guide tubes 76 and base plate 72 isoff-set and the clamping surface 77 a of clamp plate 77 is held aboutparallel to the clamping surface 36 a provided at the fixed jaw plate34; this parallel relation ship being maintained as the sliding plateassembly 70 moves along cross rods 28 of the clamp device 10.

As best seen in FIGS. 1 a and 2, a set of tension springs 80, 82 islocated between the movable jaw section 22 and the housing section 24such as to bias the sliding plate assembly 70 towards the housing boxportion 42 and cover cavity 44. This position represents the fully openclamp jaws position, and depending on the spring constants of thetension springs employed, see below, an additional locking member may berequired to secure the open position. For symmetry reasons, four tensionsprings are used, located and secured in appropriate and known manner atthe respective retention elements 84 at the corners of sliding baseplate 72 (see FIG. 5 a) and receptacle bores 83 in the housingpart/section 38 (see FIG. 4). It is noted that the number of springs,the type of springs and the spring constants of the springs used to pullthe movable jaw into the open position varies depending on severaldesign parameters set for the lifting devices such as its size andweight, etc.

The sliding plate assembly 70 also carries at its lower portion amounting axle 92 for a cam pulley 94 of the operating mechanism 90 ofthe clamp device, as will now be described with reference to FIGS. 2, 3,6 and 7 in particular.

Mounting axle (or shaft) 92 is welded onto the side of base plate 72that faces away from the fixed jaw section 20, and supports in axiallyfixed manner a cam pulley 94 for rotation. Cam pulley 94 has a discportion 97 with a peripheral groove 91, a lower circular face that isheld with small clearance parallel to the facing surface of base plate72 and a protruding hub portion 95 on the opposite face housing a pairof thrust bearings 96 that serve to support pulley 94 at shaft 92 andreact forces in axial direction of the shaft. As the cam pulley 94 maybe subjected to quite considerable loads acting in shaft direction (ieperpendicular to the extension of the pulley disc portion 97) duringclamping operation, a set of four, additional bearing wheels or rollers98 are provided to maintain the clearance gap, reduce frictional load onthe shaft bearings 96 and more evenly distribute loads past the pulley94 onto the adjoining sliding plate assembly 70. Each roller 98 isreceived in a respective one of four housing openings 99 that extendthrough the disc portion 97 and which are located equidistantly spacedapart and with equal radius about the axis of rotation represented bythe pulley shaft 92. Rollers 98 are supported at the disc portion 97 viarespective axles 101 that are press-fitted or treaded into respectiveradially extending sack bores 102 whose axis extend radially away fromand perpendicular to the pulley's axis of rotation, such that the outersurfaces of the rollers are in rolling contact with the facing surfaceof base plate 72 of the sliding plate assembly 70.

Cam pulley 90 includes, on the face spotting the hub portion 95, twoidentical cam members 105 symmetrically located about the axis ofrotation of the pulley. In plan view (as per FIG. 3) cam members 105have a curved or arcuate shape and are equiradially and concentricallyspaced from the rotation axis of the pulley. Each of the two curved cammembers 105 extends along an arc of at least 90 degrees and defines asloped guiding surface 106 (see FIG. 7 a) on its free terminal ridge. Inother words, each guiding cam surface 106 increases in height from itslowest point at 107 a, where it is at the level with the top surface ofpulley disc portion 97, to a highest point at 107 b, where it isfarthest from the surface of the pulley disc, a planar end portion 107 cof constant hight adjoining the highest point.

The slope direction of the guide surface 106, ie in clockwise oranti-clockwise direction, at the cams will be dictated by the rotationaldirection in which the pulley disc is rotated by an operating cable 110received in the pulley's groove 91 in order to displace the movable jawsection 22, ie whether the actuating cable unwinds clock oranticlockwise. In the embodiment shown in FIG. 3, cable 110 is wound tocause anti-clockwise rotation of pulley 94 upon cable 110 being pulledupwards.

As shown in FIG. 3, the upper portion of the base plate 72 of slidingplate assembly 70 carries a pair of guide rollers 87 in between whichactuating cable 110 is guided at the upper end of the assembly 70 sothat the cable extends therefrom in a line that coincides with the axisof rotation of cam pulley 94 and avoids an unsymmetrical loading of theclamp assembly 10 when hoisted (suspended). A deflection roller 88 islocated below guide rollers 87, the centre of rotation of deflectionroller 88 is vertically aligned with the right upper roller 87 because,in this embodiment, the cam rotates counterclockwise when force isexerted upwardly on cable 110 in order displace the movable jaw into itsclamping position. The upper portion of cable 110 is formed into a loopsuch that the clamp device 10 can be hooked and lifted by a liftingcrane or the like (not shown) during lifting operation. The other(lower) terminal end of actuating cable 110 is removably secured to theperimeter of cam pulley 74 at a recess using known fastening techniques.

As noted above, the operating mechanism 90 includes the actuatingrollers 86 that are mounted in fixed locations in cavity 44 ofstationary housing part 38. The location of these rollers 86 is suchthat these bear on and can travel along the bearing and guiding surface106 of the respectively associated one of the cams 105, wherein thearrangement is such that rotation of the pulley 74 causes axialdisplacement of the sliding plate assembly 70 (with the attached campulley 74) along the upper guide cross bars 28 of the clamp frame andcoaxial with the axis of rotation of the pulley. It can be seen from thearrangement of the pulley-cam mechanism that the length and the slopingangel of the cam surface maybe chosen as desired, depending on the spaceor gap between the gripping surfaces 36 a and 77 a of the fixed jaw andthe movable jaw, and thus the distance the movable jaw travels from itsopen position toward the fixed jaw in order to clamp the thinnest slabthe device is designed to grip. It should be noted also that in itsclosed position for gripping the thinnest slab the device is designed tolift, the actuating rollers 86 must not contact the cam surfaces attheir highest point 107b.

Another important point to note is that because the entire clamp device10 is lifted using the same tensile force transmitting member that isused to cause the movable jaw assembly 22 to be displaced into a sheetmaterial clamping engagement towards the fixed jaw assembly 20, alocking mechanism 60 is provided which when activated prevents the campulley 74 from being rotated and consequently keeps the jaw members 20,22 in a fixed relation to one another. The locking mechanism 60illustrated in FIGS. 2 and partly in 6, is designed to maintain themovable jaw section 22 adjacent the housing section 24 irrespective ofwhether the tension springs 80, 82 intended to bias the movable clampsection 22 into the open position are weak or strong, and irrespectiveof an actuating force being exerted on the actuating cable 110 thatwould otherwise cause the cam pulley 74 to rotate.

The locking mechanism 60 includes a locking bar 64 that is excentricallyfixed to the terminal end of the shaft of a knob 62 that extends throughand is secured in axially fixed but rotation permitting manner at thebottom wall 48 of housing part 38. Locking bar 64 is disposed toco-operate with a notched slot or groove 63 provided at the terminalfree end of cam pulley support shaft 92. The shaft 92 must hereby belong enough to allow locking bar 64 to engage slot or groove 63 of thesupport shaft when the movable jaw section 22 is in the fully openposition. When locking bar 64 is engaged in slot 64, the movable jawsection 22 is held stationary relative to the housing section 24.

An alternative locking mechanism 160 is illustrated in the clamp deviceembodiment of FIG. 8, the clamp device otherwise remaining unchanged.The manual actuating member 161 includes a spring-loaded actuator rod162 attached outside to the housing part 38. A latching plate 164 isfixed within the housing part cavity. 44 for rotation about an axisparallel to the rotation axis of the cam pulley 74. An actuator arm 163is secured rigidly to latching plate 164 and extends through an openingin the housing part 38 into engagement with the terminal end of actuatorrod 162. The cam pulley 74 has a cut-out 166 formed in its peripheryinto which the latching plate may be rotated into form-lockingengagement to prevent rotation of the pulley 74.

Operation of the clamp device will now be described. As noted above, theentire device 10 may be hoisted through actuating cable 110 beingsecured to a hoisting cable. Normally, a lifting force exerted on cable110 will cause cam pulley 74 to rotate and force the movable slidingplate assembly 70 which carries clamping plate 77 at its lower end tomove toward the fixed jaw section 20 and thus close the clamp's jaws.Depending on the type of bias springs 80, 82 used, the movable jawassembly 22 plate will or will not move when “normal” lifting force isinitially exerted on cable 110: If springs 80, 82 are relatively weak,then the cam pulley 74 of operating mechanism 90 will rotate and drivethe movable jaw toward the fixed jaw. If relatively stiff (high springconstant) springs are employed, the gravity force exerted by device 10on the cable 110 by itself will not be sufficient to overcome the biasforce of the springs when the device 10 is lifted. As such, thepulley-cam mechanism 90 will not move and the movable jaw 22 will remainopen and stationary until an actuating force is exerted that is greaterthan the weight of the device as a whole. This additional force elementcan be exerted by an operator pulling the clamping frame downwards overthe slab to be lifted

When operating the clamp device with activated locking mechanism 60,regardless of spring type employed, the locking bar or plate must bedisengaged from engagement with the cam pulley in order to allowrotation thereof and close the movable clamping jaw 22.

When the lifting clamp device 10 is located directly above the slabmaterial to be lifted, a human operator will pull the device down towardthe slab material whilst the hoop of actuating cable 110 is keptstationary. A downward directed pulling force exerted by an operatorwill causes the cam pulley to rotate, resulting in the movable jaw beingdisplaced and moved into gripping position. The entire clamp device 10is hereby lowered as the actuating cable 110 is unwound from the pulley.When stop cross bars provided below the cross bars 28 come into contactwith the upper edge of the sheet material (the width of the slabmaterial), the movable jaw 22 contacts the sheet and, together with thefixed jaw 20, grips the slab for lifting and transporting.

Once the slab material has been moved to its intended position, thelifting force is released form the cable 110 allowing the cam pulley 74,which advantageously is provided with a helical winding spring to inducerotation of the pulley in a direction counter the cable unwindingdirection, to turn and partially wind cable 110 around pulley 74,thereby allowing the movable jaw 22 to return to the open/rest positionunder the influence of the biasing springs 80, 82. The lifting operationis repeated in the same manner.

As can be seen from the above discussion, the lifting force exertedupwardly on the upper portion of the cable 110 and the centre of gravityof the slab and the lifting device are substantially aligned verticallyby inclining the movable jaw member at an angle with respect to thevertical. It has been found that an angle between 9 degrees to 11degrees with respect to the vertical gives the best results in term ofenhancing the stability of the device during lifting operation. The evendistribution of the gripping force provided by the cam mechanism and thesubstantial alignment of the lifting force and the gravity force of thedevice and slab material being lifted of the improved lifting deviceresult in the device being able to lift large and heavy slab materialwith less incident of breakage and/or deforming of the slab material,depending on whether the material is stone, marble or metal. Of course,modifications such as the profile of the cam surface of the cam members,the number and locations of the tension springs, the location of thepulleys, the size and cross-sectional shape of the cross bars, thematerial of various components of the lifting device to form a lightweight yet strong enough to withstand the heavy duty performance, etc.are all within the scope contemplated by the disclosed invention.

1. Clamping device suitable for lifting and handling of sheet-likeobjects, having a rigid frame with a fixed jaw that provides a firstclamping surface, a movable jaw that provides a second clamping surfacethat is substantially parallel to the first clamping surface, themovable jaw being supported at the frame for reciprocating, linearmovement towards and away from the fixed jaw, and an operating mechanismarranged to bias the movable jaw towards the fixed jaw in response to anactuating force being exerted on a tensile force transmitting member ofthe operating mechanism, to thereby abut the respective clampingsurfaces onto opposite faces of the sheet-like object received betweenthe jaws and frictionally clamping same against displacement,characterised in that the movable jaw comprises a sliding plate on whichthe second clamping surface is provided, the sliding plate beinginclined at an angle of about 8 to 12 degrees with respect to the firstand second clamping surfaces and being supported at the frame tomaintain said angle during movement to and from the fixed jaw, and inthat the tensile force transmitting member is guided and supported at anupper location of the sliding plate that substantially aligns with oroverhangs the second clamping surface.
 2. Clamping device according toclaim 1, wherein the inclination angle of the sliding plate is about 11degrees.
 3. Clamping device according to claim 1, wherein the operatingmechanism is a cam pulley system of the type having an actuating pulleymounted on a sliding plate with its axis of rotation perpendicularthereto, variable-height actuating cams protruding from one face of thepulley, and actuating rollers that are mounted in fixed locations on astationary frame part opposite the movable sliding plate in such manneras to co-operate with a respectively associated one of the cams therebyto travel along the cams upon rotation of the pulley, wherein thearrangement is such that rotation of the pulley causes axialdisplacement of the sliding plate and the pulley along the axis of therotation of the pulley.
 4. Clamping device according to claim 1, whereinthe operating mechanism includes a drive pulley that is mounted on thesliding plate of the movable jaw with its axis of rotation perpendicularto the plane of the sliding plate, at least four bearing rollers arearranged to support the pulley in parallel relationship at the facingsliding plate surface, at least two actuating cams are provided on thepulley so as to protrude from the pulley surface that faces away fromthe sliding plate, the at least two actuating cams being disposedsymmetrically and concentrically about the rotation axis of the pulleyand extending along an arc sector radially inwards of the pulleycircumference, the cams having a height that increases from near thepulley face to a maximum height that is related to the maximum spacingbetween the fixed and movable jaws of the clamping device in their fullyspaced apart position, the cams each defining a sloped guide and bearingsurface for respective actuator rollers that are secured in fixedrelationship on a stationary part of the clamp frame that faces thepulley, the arrangement being such that upon rotation of the pulley, theactuator rollers travel along the guide and bearing surfaces of the camsthereby displacing the pulley and therewith the associated sliding platealong the axis of rotation of the pulley relative to the rigid frame. 5.Clamping device according to claim 4, wherein the height increase in thecams is uniform and progressively increasing or progressively decreasingalong their extension.
 6. Clamping device according to claim 4, whereinthe contour of the bearing surfaces is rectilinear, curved or otherwiseso shaped as to achive during rotation of the pulley a desireddisplacement pattern of the sliding plate.
 7. Clamping device accordingto claim 3, wherein the tensile force transmitting member for rotatingthe pulley is an actuator cable that is secured to the pulley at itsperimeter and partially wound on and running in a peripheral groove ofthe pulley.
 8. Clamping device according to claim 7, wherein the rigidframe is an assembly comprising a side plate that provides the fixedjaw, a substantially rectangular parallelepiped shaped housing part withan open side disposed to face the side plate, and a plurality of, crossstruts rigidly joining the side plate and the housing part in spacedapart relationship at upper ends thereof.
 9. Clamping device accordingto claim 8, wherein pedestals at a bottom wall or diametrically opposedside walls of the housing part provide the stationary frame part thatsupports and locates the actuating rollers engaging the cams of thepulley.
 10. Clamping device according to claim 9, wherein the housingpart has a cavity in which the cam pulley of the operating mechanismlocates when the clamping device is in a fully open, non-clamping statein which the movable sliding plate comes to rest adjacent the housingpart and covers the open side face of the housing part.
 11. Clampingdevice according to claim 10, wherein the cam pulley system includes atleast four support rollers or wheels mounted at respective axles andwithin respective receptacles formed equidistantly spaced apart in thepulley, the axle extending radially with respect to the axis of rotationof the pulley, the support rollers arranged to abut a back surface ofthe inclined sliding plate on which the pulley is received.
 12. Clampingdevice according to claim 3 or 11, further including a locking mechanismthat is arranged to lock the cam pulley system against rotation. 13.Clamping device according to claim 12, wherein the locking mechanism isarranged to be engageable only when the movable jaw is in itsnon-clamping position.
 14. Clamping device according to claim 13,wherein the locking mechanism includes a rotatable locking memberoperably connected to a actuating member, the locking member beingsupported on the housing part in a location where it can be in movedinto and out of engagement with a locking receptacle provided either onthe cam pulley or a support shaft thereof.
 15. Clamping device accordingto claim 14, wherein the manual actuating member includes aspring-loaded actuator rod attached to the outside of the housing part,the locking member comprising a latching plate fixed within the housingpart cavity for rotation about an axis parallel to the rotation axis ofthe pulley, and an actuator arm extending through an opening in thehousing part into engagement with the actuator rod, the cam pulleyhaving a cut-out formed in its periphery into which the latching platemay be rotated into form-locking engagement.
 16. Clamping deviceaccording to claim 15, wherein the sliding plate has mounted to it at anupper end thereof two sheaves or guide rollers in between which isguided in abutting engagement the tensile force transmitting member incentered relationship with respect to the operating axis of the campulley system.
 17. Clamping device according to claim 16, wherein thesliding plate includes at an upper end thereof tubular guide memberstraversing the plate at about 8 to 12 degrees off-set to a perpendicularline to the plate plane, the guide members arranged to be received insliding engagement on the cross-struts that join the side plate and thehousing part.
 18. A device for lifting and handling sheet, panel andslab materials using a fixed jaw and a movable jaw for gripping thematerial to be lifted, comprising: a rigid frame having a first and asecond side arranged in spaced apart and opposed relation to each other,with each side having an upper portion and a lower portion, at least twoupper horizontal cross-bars and at least two lower cross-bars connectingthe first and second sides to form the rigid frame, the lower portion ofthe first side extends downwardly and having an inner surface and anouter surface, the inner surface being vertical and substantially flat,the lower portion of the first side forming the fixed jaw, a movableplate mounted on the at least two lower cross-bars for moving thereontoward and away from the fixed jaw, the movable plate is mounted at anangle which is equal to the angle of the lower portion of the secondside, a cam pulley mechanism for driving the movable plate along the atleast two lower cross-bars, the cam pulley mechanism comprising a pulleymounted on the movable plate for rotation, two cam members mounted onone side of the pulley for rotating therewith, and a cable wound arounda groove provided on the outer circumference of the pulley for rotatingthe pulley, the cam members each having a cam surface facing away fromthe one side of the pulley and engaging a respective roller of at leastone pair of the diametrically opposed rollers such that, when the cableis pulled upwardly, the cable rotates the pulley as the cable unwindsfrom the pulley, the cam surfaces rotate and ride on the at least onepair of diametrically opposed rollers to drive the movable plate and themovable jaw toward the fixed jaw while the angle of the movable plate ismaintained the same.
 19. A device according to claim 18, wherein thepulley further comprising at least four rollers mounted radiallyinwardly of the groove of the pulley and in contact with a surface ofthe movable plate to reduce friction when the pulley rotates relative tothe movable plate.
 20. A device according to claim 18, wherein tensionsprings are provided to pull the movable plate toward the second side.21. A device according to claim 18, wherein a locking device mounted tothe second side and comprising a locking plate and handle for rotatingthe locking plate into engagement with a groove in a support shaft ofthe cam pulley mechanism to lock the cam pulley mechanism fromdisplacing relative to the second side.